1. Field of the Invention
The present invention is directed to a planar image detector for electromagnetic radiation of the type having radiation-sensitive pixel elements arranged in a matrix that has a surface with active pixels for imaging and a surface with correction pixels, not exposed to the radiation, for generating correction signals, and having contact surfaces next to the surfaces with the pixels.
2. Description of the Prior Art
Image detectors of the above type are utilized, for example, in X-ray diagnostic devices as shown in FIG. 1.
FIG. 1 shows an X-ray diagnostic apparatus known from German PS 195 27 148 having an X-ray tube 2, supplied with high-voltage and filament voltage by a voltage generator 1, that generates a conical beam of X-rays 3 that penetrates a patient 4 and generates radiation images on a solid-state detector 5 that is sensitive to the X-rays 3. The output signal of the solid state detector 6 is supplied to an image system 7. The imaging system 7 can include converters, image memories and processing circuits. The output of the imaging system 7 is supplied to a monitor 8 for playback of the acquired X-ray images. Operating elements 9 are connected via a system control and communication unit 10 to the other components of the X-ray diagnostic apparatus.
FIG. 2 shows a known solid-state detector 5 in a perspective cross-section. The core component of the solid-state detector 5 composed of a solid-state pixel matrix, line drivers and amplifiers. The solid-state pixel matrix is in turn composed of a layer with a scintillator 11 of, for example, cesium iodide (CsI) that, when irradiated by the X-rays 3, supplies visible photons to a pixel matrix 12 of amorphous silicon that produces a visible X-ray image. As shown enlarged in FIG. 2, each of the pixels or picture elements of this pixel matrix 12 is composed of a photodiode 13 and a switch that is connected to row lines 15 and column lines 16. The pixel matrix 12 is applied on a glass substrate 20.
All pixels of a line are simultaneously addressed by the line drivers 17 and are read out. The signals are processed in parallel in a number of amplifiers 18. In the simplest case, an image is progressively read out line-by-line.
Two undesired components (xe2x80x9coffset signalsxe2x80x9d) add to the desired signals. The first component varies from pixel-to-pixel. The second component is the same for all pixels of a line but varies from line-to-line and from frame-to-frame. This so-called line noise is very disturbing since the eye is highly receptive to extended gray scale differences.
German PS 195 27 148 discloses an arrangement for correcting the line signal (xe2x80x9cline noise correctionxe2x80x9d) in an a-Si panel, wherein an unirradiated edge zone lies at the left image edge, i.e. the line start, in a radiographic solid-state detector, for example an a-Si detector. This dark reference zone (DRZ) 22 shown in FIG. 3 herein is covered in a light-tight and radiation-tight manner by a lead shielding 21, so that no additional signal arises in the bright image. The signal values of the unilluminated pixels are utilized for correction of the line signal. They serve the purpose of reducing the line noise and correcting small transients in the offset.
The output signal of a line is shown over the column number n in FIG. 4. The signal S is composed only of the line offset 23 in the region of the DRZ 22. This is determined by averaging the values of the individual, occluded pixels of a line. These line offsets 23 are then subtracted from the signal values of all pixels of the same line.
In the known solid-state detector 5 schematically shown in FIG. 5, the active surface 24 should extend as far as possible to the outside edge of the housing 25, so that the edge region 26 that is not usable for imaging, is as small as possible. The area of the pixels 28 for the DRZ 22 shown in FIG. 6 can have a width of, for example, 5-15 mm and thus considerably enlarges the distance of the active surface 24 from the outside edge of the housing 25, and thus the edge region 26.
In order for pixels to be used as DRZ pixels 28, they must be driven by the same channels of the line drivers 17 as the pixels to be corrected. Insofar as possible, further, they must exhibit the same properties as the active pixels 29 to be corrected. This is assured when the DRZ pixels 28 are manufactured on the same glass substrate 20 with the same production steps such as a-Si:H deposition, photolithography, etching processes, etc.
A typical manufacturing procedure for an a-Si pixel matrix is explained on the basis of FIG. 6. Contact surfaces 30, as well as a surface (28 and 29) composed of pixels with photodiode 13 and switch 14 are produced on the glass substrate 20. The areas of the active pixels 29 and those of the pixels 28 for the DRZ 22 do not differ in terms of their structure. The difference between the active pixels 29 and the pixels 28 for the DRZ 22 arise only by covering with the lead shielding 21 and/or by non-coating with scintillator material according to FIG. 3. The saw lines 31 (kerfs) indicate how the pixel matrix 12 is sawn from the glass substrate 20.
U.S. Pat. No. 4,678,938 discloses a solid-state image converter with a control loop for the voltage of the line selection signal dependent on the output signal from the pixels of a dark reference zone allocated to the corresponding line, so that the output signal of the active pixels becomes free of offset and dark current.
European Application 0 838 859 discloses a photoelectric conversion unit having signal correction for generating, in particular, moving images, wherein some pixels are covered by shielding against X-rays and light.
An object of the present invention is to provide a planar image detector of the type initially described wherein the image-active area is optimally large in relationship to the overall area, so that the image-inactive edge regions are as narrow as possible.
This object is inventively achieved in a planar image detector wherein the area with the correction pixels is arranged in a different plane from the area with the active pixels and at one of the sides of the image detector with the contact surfaces of the active pixels. The contact surfaces can be arranged separately from the imaging part of the image detector or on the back side thereof.
It has proven advantageous when the image detector is implemented in two pieces, with a first detector part with the correction pixels being arranged in front of or behind a contact surface of the active pixels of a second detector part, in the radiation propagation direction.
The image-inactive edges become especially small when the first detector part with the correction pixels is arranged perpendicularly to the second detector part with the active pixels.
An identical structure of the two detector parts is obtained by sawing them both from a substrate on which they are manufactured in common.
As a further alternative, the image detector can be inventively arranged on a substrate, and the area with the correction pixels can be arranged following one of the contact surfaces of the active pixels in the radiation direction on that side of the substrate lying opposite the active pixels.
Inventively, the area with the correction pixels can be covered from radiation by a lead shielding and/or only the area with the active pixels can be covered with a scintillator for converting X-rays into visible light.
The layer with the pixels can be composed of amorphous silicon (aSi, aSi:H).